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Milky Way Dark Matter Object Detected For First Time

07.12.2001

Astronomers from the University of Pennsylvania, in collaboration with an international team of researchers, have made the first direct detection and measurement of the properties of a dark matter object in the Milky Way.
This observation of a gravitational microlensing event -- a temporary increase in the brightness of a background star during the time it takes dark matter to pass in front of it -- is reported in today’s issue of Nature.

"By measuring its mass, distance and velocity, we have established the first complete picture of a massive compact halo object, or MACHO," said co-author Charles R. Alcock, professor of physics and astronomy at Penn. "This demonstrates that microlensing light data, high-resolution images and spectroscopy should allow astronomers to characterize a significant fraction of the Milky Way’s dark matter."

Alcock, who serves as lead researcher on the international MACHO Project, made much of his contribution to the work in his previous capacity as director of the Institute of Geophysics and Planetary Physics at the Lawrence Livermore National Laboratory in California.

The team used the Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope to take images and make spectra of a MACHO microlens, making it possible to determine the mass of the MACHO and its distance from the Earth. In this case, the MACHO is a star 600 light-years away with a mass 5 to 10 percent the mass of the sun, making it a dwarf star and a faint member of the disk population of stars in the Milky Way.

"For the first time, we’ve been able to determine the detailed characteristics of a lens," said Cailin Nelson, a University of California at Berkeley graduate student working at Livermore with the MACHO team. "This shows that we will be able to determine the makeup of MACHOs and their role in the universe. We expected about one of our microlenses to belong to the normal, stellar component of the Milky Way, and it just happened that this was the one."

"In order to observe and then follow up more unusual microlensing events such as this one, we need to find many more events," said Kem Cook, the Livermore team leader. "We are just beginning a new five-year microlensing survey which should yield the number of events we need to identify the nature of the main microlensing population."

For the past 10 years, active search projects have looked for possible candidate objects for dark matter. One of the many possibilities is that the dark matter consists of atomic-sized weakly interacting massive particles, or WIMPs. Another possibility is that the dark matter consists of MACHOs, such as dead or dying stars (neutron stars and cool dwarf stars), objects similar to stars but too small to "light up" (planets and brown dwarfs), or black holes of various sizes.

Previous research has shown that if some of the dark matter were in the form of MACHOs, its presence could be detected by the gravitational influence MACHOs would have on light from distant stars. If a MACHO passes in front of a star in a nearby galaxy, such as the Large Magellanic Cloud, then the gravitational field of the MACHO will bend the light and focus it into telescopes.

The MACHO acts like a gravitational lens and causes the brightness of the background star to increase for the short time it takes for the MACHO to pass by. Depending on the mass of the MACHO and its distance from the Earth, this period of brightening can last days, weeks or months.

Gravitational lensing can also be observed on much larger scales around large mass concentrations, such as clusters of galaxies. Since MACHOs are much smaller, they are referred to as "microlenses."

The form and duration of the brightening caused by the MACHO can be predicted by theory and searched for as a clear signal of the presence of MACHO dark matter. But in a normal event, the brightening alone is not enough information to yield the distance to the MACHO, its mass and velocity as independent quantities. It is only for unusual events, such as this one, that more can be learned.

In 1991, a team of astronomers from Livermore, the Center for Particle Astrophysics at the University of California at Berkeley and the Australian National University joined forces to form the MACHO Project. This team used a dedicated telescope at the Mount Stromlo Observatory in Australia to monitor the brightness of more than 10 million stars in the Large Magellanic Cloud over a period of eight years.

The team discovered its first gravitational lensing event in 1993 and has now published approximately 20 examples of microlenses toward the Magellanic Clouds. These results demonstrate that there is a population of MACHO objects surrounding the Milky Way galaxy that could comprise as much as 50 percent of the total dark matter content.

The MACHO collaboration is made up of Alcock and Matthew J. Lehner at Penn; K.H. Cook, A.J. Drake, S.C. Keller, S.L. Marshall, C.A. Nelson and P. Popowski of Livermore; R.A. Allsman of the Australian National Supercomputing Facility; D.R. Alves of the Space Telescope Science Institute; T.S. Axelrod, K.C. Freeman and B.A. Peterson of the Mount Stromlo Observatory; A.C. Becker of Bell Labs; D.P. Bennett of the University of Notre Dame; M. Geha of the University of California at Santa Cruz; K. Griest and T. Vandehei of the University of California at San Diego; D. Minniti of Universidad Catolica; M.R. Pratt, C.W. Stubbs and A.B. Tomaney of the University of Washington; P.J. Quinn of the European Southern Observatory; W. Sutherland of the University of Oxford; and D. Welch of McMaster University.

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